Even if Chula Vista manages to close much of its estimated $18 million budget gap next year with large concessions from five union groups, city officials said they will still need to lay off 50 employees.

Those staffing cuts would come after four years of belt-tightening. Since 2007, Chula Vista has eliminated 259 full-time positions and trimmed $40 million from the budget.

City Manager Jim Sandoval said the next round of layoffs, whether it’s 50 or more, will likely be made in January. The city hopes to realize additional savings in advance of the 2011 fiscal year, which begins in July.

Chula Vista’s projected deficit for next year amounts to 13.5 percent of its $133 million annual operating budget. The initial forecast of a $12.5 million shortfall grew to $18 million with the Nov. 2 defeat of Proposition H, which would have expanded the city’s utility users tax.

City officials originally proposed laying off 71 employees, including 33 police officers, and carrying out other downsizing measures. After the November election, the layoff target has grown to 146 employees if the unions refuse to make concessions.

Many of the layoffs would affect public-works and library employees, Sandoval said, while the downsizing cap for the Police Department would remain at 33.

He is urging union members to pay their full share of employee costs for the pension system and forgo raises included in their current contracts. He said discussions with the unions are ongoing.

“Frankly, we’re trying to get the full amount,” Sandoval said. “If it’s the full amount, we would be able to clear $12.5 million from the deficit.”

Three people poised to form a new majority bloc on the City Council have called on the city to look at other cost-cutting options in addition to pension reform.

Councilmen Steve Castaneda and Rudy Ramirez would like to explore cuts to executive compensation. In a memo last month, they asked Sandoval to roll back all raises given to nonunion employees in the past three years. They also want a more thorough look at staffing levels and further consolidation of departments, among other things.

“Frankly, our request was swept under the rug to convince voters that the only way to balance the budget was through drastic cuts to public safety and public services unless (Proposition H) was approved by voters,” Castaneda said.

In a memo dated Monday, Sandoval outlined the reasons he couldn’t support a salary rollback for nonunion workers. He said most of the compensation boosts during the past few years have been tied to changes in job assignments, cost-of-living adjustments and step increases — salary bumps set by Civil Service rules.